Simple interactive command shell.

This is meant to be used as a fallback or recovery shell, something that
would work no matter the state of the system is and let the user enter
simple commands to fix (or reboot or install) the system.

The shell supports basic line editing, tab completion and non-persistent
history. Skipping either of those makes the shell really uncomfortable to
work in. Non-interactive use and any sort of scripting is not supported.

See ../../doc/shells.txt for more info.
